Seals used in the petroleum, chemical and transportation industries must meet stringent performance requirements for sealability, reliability and durability. Seals are used for rotary and reciprocating valves, pistons and rods, and in other applications such as in packing rings, piston rings, and gland structures in industrial equipment such as pumps and compressors.
A particularly useful seal for a valve stem which has received widespread acceptance in the industrial marketplace is the U-cup seal with an expander ring. This type of seal includes a gland member having an annular base of generally rectangular cross-section, and a pair of (typically symmetrical) sealing lips formed in one piece with the base and extending outwardly from one side thereof. The sealing lips define an annular U-shaped cavity for the expander ring. The expander ring is disposed within the cavity and the sealing lips partially surround and retain the expander ring. The expander ring has a peripheral dimension which is slightly larger than the cavity such that a slight outward pre-load is provided through the sealing lips against the valve stem and valve housing when the seal is assembled around the valve stem.
One type of seal assembly utilizing a U-cup type seal is the "EC Pak", which is available from the assignee of the present invention. This seal assembly is specifically designed as a modular sealing system to control fugitive emissions of MTBE/blended gasoline or other fluids around a valve stem. The "EC Pak" includes i) one or more primary U-cup seal rings formed from a relatively rigid, thermoplastic material (e.g., PTFE, PEEK, PFA) with a temperature range from cryogenic to +500 degrees Fahrenheit and resistance to a wide range of chemicals: ii) a single, relatively flexible secondary seal ring located downstream from the primary seal ring(s) as a backup to the primary seal ring(s) and which is formed from molded synthetic rubber; and iii) a nose or backup ring, formed from a relatively rigid thermoplastic material, which is then located at the upstream and/or downstream end of the seal stack to protect the primary and secondary seal rings and prevent extrusion. The EC Pak has been found to perform satisfactorily for a valve stem seal under difficult conditions because of the flexible, self-adjusting nature of the seal stack conforming to the gap between the components, and the resistance of the primary and secondary seal rings to the corrosive or detrimental effects of the fluid, the temperatures within the valve, and the movement of the valve stem. For example, in a dynamic seal test where the fluid media consisted of a 45% aromatic blend (20% MTBE, 34% Toluene, and 46% unleaded regular gasoline), the EC Pak performed satisfactorily for over 25,000 duty cycles with only minimal 5 ppm leakage observed around a 11/2" valve stem.
In the past, it is believed that the dimensions for the lips of the gland member in a U-cup type seal and for the expander ring have typically been determined using a "50% rule of thumb" method. That is, it is believed that seal designers have typically measured the cross-section of the sealing gap between the components to be sealed, and calculated 50% of this width as an appropriate maximum diameter for the expander ring. The gland lips on either side of the expander ring would then each account for about 25% of the remaining dimension. For example, for a 0.250 inch gap between components, a designer might typically choose an expander ring which has about a 0.125 inch diameter. In fact, for such a gap, it has been known to choose a 0.139 inch O-ring, which is a common O-ring dimension closest to this 50% value. Similarly, for a 0.500 inch gap, a designer might calculate 50% of this (0.250 inches), and choose an O-ring of a 0.275 inch diameter, which is the closest common O-ring dimension to this value. The "50% rule of thumb" method is believed to be based on the seal designer's belief that for typical gland members (which are usually formed from a resilient material), it is necessary to provide a substantial thickness to the lip seals for wear purposes. Such U-cup seals with these dimensional characteristics are shown in Felt, U.S. Pat. No. 3,169,776; Knudson, U.S. Pat. No. 3,342,500; and Ksieski, U.S. Pat. No. 3,645,543, among others.
It is believed that designers up to now have been satisfied using the above method to provide a seal with an acceptable level of durability, sealability and reliability for most industrial applications. However, for more demanding requirements it has been found that this method is not always acceptable. That is, for applications where a primary seal ring is formed from a material which is resistent to a wide range of chemicals, and can withstand elevated temperatures, for example Teflon, the "50% rule of thumb" method can have certain drawbacks, and in any case does not maximize the performance potential of the primary seal ring. It is believed that this due in part to the chemically resistant and temperature resistant gland materials being relatively rigid, which leads to relatively inflexible sealing lips and a relatively non-resilient expander ring which can thereby reduce the cycle duty of the seal, provide a seal which is limited in conforming to irregularities in the valve stem and valve housing, and can create heat and/or assembly problems between the components as the valve stem moves, thus reducing the overall performance characteristics of the seal. As such, while some of the above-described seals have been found to function satisfactorily in certain applications for the petroleum; chemical and transportation industries, it is believed that there is a demand in the industry for a seal with even greater performance characteristics.